UNION 联合查询

1.UNION ALL联合查询

同样为了演示方便，先向 teacher 表插入多条测试数据：

INSERT INTO teacher
(name,age,id_number,email)
VALUES
('姓名一',17,'42011720200604077X',NULL),
('姓名二',18,'42011720200604099X','123@qq.com'),
('姓名三',19,'42011720200604020X',NULL),
('姓名四',20,'42011720200604022X','345@qq.com'),
('姓名五',21,'42011720200604033X',NULL),
('姓名六',22,'42011720200604077X',NULL),
('姓名七',23,'42011720200604099X','123@qq.com'),
('姓名八',24,'42011720200604020X',NULL),
('姓名九',25,'42011720200604022X','345@qq.com'),
('姓名十',26,'42011720200604033X',NULL),
('姓名1',27,'42011720200604077X',NULL),
('姓名2',28,'42011720200604099X','123@qq.com'),
('姓名3',29,'42011720200604020X',NULL),
('姓名4',30,'42011720200604022X','345@qq.com'),
('姓名5',31,'42011720200604033X',NULL),
('姓名6',32,'42011720200604077X',NULL),
('姓名7',33,'42011720200604099X','123@qq.com'),
('姓名8',32,'42011720200604020X',NULL),
('姓名9',31,'42011720200604022X','345@qq.com'),
('姓名0',30,'42011720200604033X',NULL)

执行结果如下图：

现在使用 UNION ALL 把满足两种查询条件的结果集并到一起：

SELECT * FROM teacher WHERE age > 20
UNION ALL
SELECT * FROM teacher WHERE age > 25;

执行结果如下图:

2.UNION 联合查询

现在使用 UNION 把上面两种结果集并到一起：

SELECT * FROM teacher WHERE age > 20
UNION
SELECT * FROM teacher WHERE age > 25;

执行结果如下图:

3.小结